1. Field of the Invention
This invention relates to rotation control systems, and more particularly to rotation control systems capable of controlling the speed and phase of rotation of a rotator.
2. Description of the Prior Art
In the following this specification is described by taking, for example, a system for controlling the operation of a rotary cylinder used in the rotary head type magnetic video recording and reproducing apparatus (hereinafter referred to as "VTR").
FIG. 1 illustrates an example of the system for controlling the operation of a rotary cylinder as the rotator in VTR. There are shown a magnetic tape 1, a magnetic head 2 for recording and reproducing video signals, a rotary cylinder 3 holding the magnetic head 2, an electric motor 4 for driving the cylinder 3, a magnet 5 for phase detection incorporated in the motor 4, a head 6 for phase detection by detecting the position of the magnet 5, a signal generator 7 for producing a signal representing the frequency proportional to the number of revolutions of the motor 4, a video signal inlet 8, a vertical-hold separation circuit for separating vertical synchronizing signals from the video signals, a control head 10 by which when recording the video signals the vertical synchronizing signals separated by a vertical-hold separation circuit 9 are recorded as the control signal, and when reproducing the video signals, the control signal is reproduced, a control track 11 on which the control signal is recorded, changeover switches 12 and 13, a square wave forming circuit 14, a trapezoidal wave forming circuit 15, a pulse shaping circuit 16, a delay circuit 17, a charging and discharging circuit 18, sample and hold circuits 19 and 20, and a control circuit 21 for amplifying the outputs of the sample and hold circuits 19 and 20 in appropriate combination.
FIG. 2 is wave form diagrams drawn to a common time scale representing signals appearing at various points, (a) through (j), in the system of FIG. 1. The operation will next be described below. With regard first to the speed control system, the output (a) of the signal generator 7 is applied to the pulse shaping circuit 16, and the pulse shaping circuit produces a pulse signal (b) in synchronization with the output (a). The pulse signal (b) is applied as sampling pulses to the sample and hold circuit 20 and also to the delay circuit 17 in which the input signal is delayed by a minute time. The output of the delay circuit 17, or the delayed pulses (c) are applied as trigger pulses to discharge the charging and discharging circuit 18. The charging and discharging circuit 18 comprises a charging current source, a condenser, and a discharging circuit to be discharged when the trigger pulses are given thereto. Then, the output (d) of the charging and discharging circuit 18 is applied to the sample and hold circuit 20 where sampling is performed by the aforesaid pulse signal (b). Then, the output (e) of the sample and hold circuit 20 is applied as a speed error voltage to the control amplifier 21.
Next, explanation is given to the phase control system. The PG head 6 produces a pulse signal (f) (hereinafter referred to as "PG pulses") in synchronization with the motor 4, which is then applied to the square wave forming circuit 14. The square wave forming circuit 14 produces square waves (g) with their leading and trailing edges synchronized with the positive and negative going PG pulses (f) respectively. The trapezoidal wave forming circuit 15 produces trapezoidal waves (h) in synchronization with the square waves (g), and the trapezoidal waves (h) are applied to the sample and hold circuit 19. On the other hand, the vertical synchronizing signal separated from the video signal by the vertical-hold separation circuit 9 is recorded as the control signal in the control track on the magnetic tape 1 through the control head 10 when recording the video signal, and is also applied as the sampling pulses (i) to the sample and hold circuit 19. Also when reproducing the video signal, the thus-recorded control signal is reproduced from the control track, and the reproduced control signal is applied as the sampling pulse (i) to the sample and hold circuit 19. In the sample and hold circuit 19, the trapezoidal waves (h) are sampled by the aforesaid sampling pulses (i). Then, the output of the sample and hold circuit 19 is applied to the control amplifier 21.
FIG. 3 is an electrical circuit diagram illustrating an example of a conventional control amplifier 21 in the system of FIG. 1. In FIG. 3, Reference symbols A1-A3 identify operational amplifiers; reference symbols R, R1 and R2 identify resistors; reference symbol Q identifies a transistor. Also in the following, an output voltage of the operational amplifier A1 is denoted as V1, an output voltage of the operational amplifier A2 is denoted as V2, and an output voltage of an emitter follower including the transistor Q and the operational amplifier A3, which is applied to a motor 4, is identified as V.sub.0. This motor 4 is the same as that illustrated in FIG. 1.
The operation of this control amplifier 21 is as follows: The output (e) of the sample and hold circuit 20 and the output (j) of the sample and hold circuit 19 are applied as a speed control voltage (hereinafter referred to as "Vs") and a phase control voltage (hereinafter referred to as "Vp") respectively to terminals 22 and 23. To a terminal 25 there is provided a constant voltage Vcc, and to a terminal 24 there is provided a half of this constant voltage Vcc, or constant voltage 1/2Vcc. FIG. 4 is a characteristic curve representing the relationship between the number of revolutions (hereinafter referred to as "N") of the cylinder 3 and the speed control voltage Vs. It is so set that when the cylinder 3 is rotating at a prescribed speed (hereinafter referred to as "N.sub.0 "), the speed control voltage Vs becomes 1/2Vcc. Also FIG. 5 is a characteristic curve representing the relationship of the phase difference (hereinafter referred to as ".theta.") between the phase of rotation of the cylinder 3 and the aforesaid vertical synchronizing signal or reproduced control signal with the phase control voltage Vp. In FIG. 5, the starting point in time .theta..sub.0 is taken at a prescribed value (hereinafter referred to as ".theta..sub.0 ") of the aforesaid phase difference. It is at this time that the phase control voltage Vp becomes 1/2Vcc. It is set such that the motor 4 has the number of revolutions N.sub.0 when the voltage applied thereon is 1/2Vcc. Here the number of revolutions V.sub.0 is expressed by the following formula in terms of the speed control voltage Vs and the phase control voltage Vp: ##EQU1## Here when Vs=1/2Vcc and Vp=1/2Vcc, that is, when the drum 3 rotates with the phase difference .theta..sub.0 and the number of revolutions N N.sub.0, from formula (1) we have V.sub.0 =1/2Vcc. Thus, the number of revolutions of the motor 4 is adjusted to that prescribed value. If the number of revolutions N increases from N.sub.0 by .DELTA.N, or the speed of rotation becomes faster, as .DELTA.Vs/.DELTA.N=-a is assumed at N=N.sub.0 in FIG. 4, it follows from formula (1) that ##EQU2## In other words, the voltage V.sub.0 applied to the motor 4 lowers with the result that the number of revolutions N decreases to N.sub.0. And in similar analogy, if the number of revolutions N is decreasing, the number of revolutions N tends to increase to N.sub.0. Next, if the phase .theta. is in advance .DELTA..theta. from .theta..sub.0, as .DELTA.Vp/.DELTA..theta.=-b is assumed at .theta.=.theta..sub.0 in FIG. 5, it follows from formula (1) that ##EQU3## In other words, the voltage V.sub.0 is applied to the motor 4 lowers. As a result, the phase .theta. tends to delay approaching .theta..sub.0. And in a similar analogy, if the phase .theta. is in delay, the phase .theta. tends to advance to .theta..sub.0. In such a manner, the speed and phase of rotation are controlled.
Let us next explain the transient characteristics at the starting time. We will assume that the motor 4 is initially at a standstill and then actuated therefrom to start rotation. As the number of revolutions of the motor 4 increases, the speed control voltage Vs approaches 1/2Vcc. And during this time, for the speed control voltage Vp, there takes place large variations in the voltage with frequencies equal to the differences between the instantaneous angular frequency of the cylinder 4 and the frequency of the sampling pulses (i). In general, however, the resistor R1 is larger than the resistor R2. That is, the gain of the speed control system is larger than that of the phase control system. Therefore, it may be considered that after the start of motion of the cylinder 3 the speed control system mainly works until the number of revolutions N reaches N.sub.0.
Assuming that when the number of revolutions nears N.sub.0, the .theta. goes an angle .DELTA..phi. ahead of the .theta..sub.0, then if the resultant voltage V.sub.0 applied on motor 4 takes the following value derived from formula (3), or ##EQU4## no problem will arise. On the other hand, however, lowering of the voltage V.sub.0 of formula (4) on the motor (4) causes the number of revolutions N to increase. If the number of revolutions N is increased by .DELTA.n, formulae (2) and (4) give ##EQU5## This implies that the displacement of the speed control voltage Vs opposite in sign to the displacement of the speed control voltage Vp is produced so that the displacement of the voltage V.sub.0 on the motor 4 becomes smaller. In conclusion, the time interval from the moment at which the number of revolutions N has reached N.sub.0 to the moment at which the phase .theta. reaches .theta..sub.0 is elongated. Also because of the external disturbance, a control oscillation results.
As has been described above, when in controlling the speed and phase of rotation of a rotator, it was difficult to establish the prescribed phase value of rotation soon after the speed of rotation became constant.
With the aforesaid problem in mind, it is an object of the present invention to provide a rotation control system for controlling the operation of a rotator to reach a given speed of rotation and further therefrom to reach a given phase of rotation during a reduced time from the start of rotation, and for ensuring good stability of control system operations.
Another object of the invention is to provide a VTR capable of starting recording or reproduction soon after the start of rotation of a rotary cylinder holding a head.
The additional objects and features of the present invention will become apparent from the following description of embodiments thereof by reference to the accompanying drawings.